This invention relates to improvement in gear assemblies, and in particular to power assisted steering assemblies which incorporate a worm and wheel gear assembly for transferring torque from an electric motor to a steering column or output shaft operatively connected thereto.
It is known to provide a power steering system for a vehicle comprising an electric motor having a stator and a rotor, an input shaft operatively connected to the rotor and adapted to rotate therewith, an output shaft associated with a steering column, and a gearbox adapted to transfer torque from the input shaft to the output shaft in response to a measure of torque in the output shaft produced by a torque sensor. The motor is typically operated to apply an increasing torque to the output shaft as the measured torque increases, thus applying an assistance torque which helps to steer the vehicle.
Steering systems which use a worm and wheel gearbox to transfer torque from the input shaft to the output shaft (the worm usually provided on the input shaft and the wheel on the output shaft) are well known in the art. One problem with these systems is rattle between the worm and wheel which is especially noticeable to the driver when driving on rough roads or when the steering wheel is swayed from side to side under low load. If the worm and wheel teeth are not fully meshed at all times, noise may be produced which may be heard by the driver. This incorrect meshing may arise due to manufacturing tolerances, thermal changes in dimensions, distortion due to torsional loads and wear during service. Various attempts have been made to overcome this problem. For example, a resilient biasing means in the form of an L-shaped leaf spring may be provided which acts between the housing and a support bearing means which supports the end of the worm shaft furthest from the motor. The L-shaped spring applies a light biasing load to the bearing means to keep the worm and wheel engaged.